A cold cathode fluorescent lamp (CCFL) has been used as a backlight light source of a liquid crystal display such as a liquid crystal television set or a liquid crystal monitor of a notebook personal computer. This CCFL generally has higher efficiency and longer life than those of a normal hot cathode fluorescent lamp and then, a filament which the hot cathode fluorescent lamp has is omitted.
A high AC voltage is required in order to start and operate this CCFL. For example, a starting voltage is about 1000 V and an operating voltage is about 600 V. This high AC voltage is generated from a DC power source of a notebook personal computer, a liquid crystal television set, etc. using an inverter.
Conventionally, a Royer circuit has generally been used as an inverter for CCFL. This Royer circuit is constructed of a saturable magnetic core transformer, a control transistor, etc., and then self-oscillates by characteristics of non-linear current gain of the control transistor and non-linear magnetic permeability of the saturable magnetic core transformer. The Royer circuit itself does not require an external clock or a driver circuit.
However, the Royer circuit is basically a constant-voltage inverter, and cannot maintain a constant output voltage when an input voltage or a load current varies. Therefore, a regulator for supplying electric power to the Royer circuit is required. Because of this, it is difficult to miniaturize the inverter using the Royer circuit and also, electric power conversion efficiency is low.
An inverter using a center tap type transformer having a primary winding in which a DC voltage is supplied to a center tap and a secondary winding for AC voltage output has been proposed (see JP-T-2002-500427 and JP-A-6-14556).
In the inverter of JP-T-2002-500427, a DC voltage is supplied to a center tap of a primary winding and semiconductor switches are respectively had between a ground and each end of the primary winding and their semiconductor switches are alternately turned on and off. A PWM control apparatus for performing PWM control of a DC voltage supplied to its inverter is disposed. Then, electric power supplied from the inverter to a load is controlled by control of a DC current by the PWM control apparatus.
The inverter of JP-A-6-14556 comprises a step-up transformer having a primary winding in which a DC power source is connected to a center tap, a secondary winding for AC voltage output and a tertiary winding for feedback, a resonance capacitor which is connected between both ends of the primary winding of its step-up transformer and constructs an LC resonance circuit between the resonance capacitor and an inductance of this primary winding, a pair of semiconductor switches alternately turned on and off by an output voltage of the tertiary winding, one end sides of the semiconductor switches being respectively connected to the different ends of its resonance capacitor and the other end sides being earthed, and a variable inductor connected to the inside of its LC resonance circuit. Then, an output voltage of the inverter is controlled by controlling an inductance of the variable inductor.
Means using a Royer circuit has problems that it is difficult to miniaturize the means and also conversion efficiency is low. In the inverter of JP-T-2002-500427, the PWM control apparatus for performing PWM control of a DC voltage supplied to its inverter is required in addition to the inverter, so that a structure as the whole DC-AC conversion apparatus becomes complicated and also it is difficult to miniaturize the inverter. Also, the inverter of JP-A-6-14556 comprises the variable inductor connected to the inside of the LC resonance circuit and an output voltage is controlled by controlling its inductance, so that a structure becomes complicated and also it is difficult to miniaturize the inverter.
Further, with an increase in screen size of a liquid crystal display such as a liquid crystal television set or a liquid crystal monitor of a notebook personal computer, plural CCFLs have been distributed and arranged as a backlight light source. In this case, light from the plural CCFLs mutually interferes and becomes a cause of flicker etc., so that it becomes necessary to synchronously turn on each of the CCFLs in the same phase.
For this purpose, it is contemplated to construct an inverter of a discrete circuit and supply AC power of the same phase to plural CCFLs.
However, by reasons that it is necessary to reduce an influence on other apparatus by decreasing a routed distance of high-voltage wiring to the CCFL and also that parasitic capacitance of the CCFL is effectively used in resonance with a transformer, it is desirable that the inverter for controlling each of the CCFLs be arranged as close as possible to the CCFLs.
JP-T-2002-500427 and JP-A-6-14556 are seen as the related art.
DISCLOSURE OF THE INVENTION
Problems that the Invention is to Solve
An object of the invention is to provide a DC-AC conversion apparatus (an inverter) for generating an AC voltage for driving a load from a DC power source, a DC-AC conversion apparatus suitable for parallel running of plural loads, the DC-AC conversion apparatus capable of finely adjusting electric power supply to the loads by a simple configuration using a transformer having a primary winding with a center tap to which a DC voltage is supplied, a controller IC used in the DC-AC conversion apparatus, and a parallel running system for synchronously controlling plural DC-AC conversion apparatus in the same phase.
Means for Solving the Problems
The invention provides a controller IC for controlling a first semiconductor switch and a second semiconductor switch for driving a load, having an oscillator block for generating a first triangular wave signal and a second triangular wave signal having a relation in which the first triangular wave signal is reversed when a capacitor for frequency decision and a resistor for frequency decision are connected, a pulse width modulation circuit for comparing the first triangular wave signal with a feedback signal formed based on a current flowing through the load and generating a first pulse width modulation signal and comparing the second triangular wave signal with the feedback signal and generating a second pulse width modulation signal, and a driving signal output block for alternately outputting a first switch driving signal for turning on the first semiconductor switch based on the first pulse width modulation signal and a second switch driving signal for turning on the second semiconductor switch based on the second pulse width modulation signal, characterized in that the driving signal output block generates the first switch driving signal and the second switch driving signal at timing at which an off period during which both of the first semiconductor switch and the second semiconductor switch are turned off is set between a period for which the first semiconductor switch is in the on state and a period for which the second semiconductor switch is in the on state.
In the controller IC, the oscillator block to which the capacitor for frequency decision and the resistor for frequency decision are not connected generates a triangular wave signal supplied from the outside and a triangular wave signal having a relation in which the triangular wave signal is reversed.
In the controller IC, the driving signal output block outputs the first switch driving signal from a point in time of one vertex of the first triangular wave signal to a point in time when the first triangular wave signal immediately after the point in time becomes equal to the feedback signal, and outputs the second switch driving signal from a point in time of the one vertex of the second triangular wave signal to a point in time when the second triangular wave signal immediately after the point in time becomes equal to the feedback signal.
The invention also provides a DC-AC conversion apparatus having a transformer having a primary winding with a center tap and at least one secondary winding, the center tap being connected to a first potential point of a DC power source, a first semiconductor switch connected between one end of the primary winding and a second potential point of the DC power source, a second semiconductor switch connected between the other end of the primary winding and the second potential point, a current detection circuit for detecting a current flowing through a load connected to the secondary winding, an oscillation circuit for generating a first triangular wave signal and a second triangular wave signal having a relation in which the first triangular wave signal is reversed, a pulse width modulation circuit for comparing the first triangular wave signal with a feedback signal formed based on a current detected by the current detection circuit and generating a first pulse width modulation signal and comparing the second triangular wave signal with the feedback signal and generating a second pulse width modulation signal, and a driving signal output circuit for alternately outputting a first switch driving signal for turning on the first semiconductor switch based on the first pulse width modulation signal and a second switch driving signal for turning on the second semiconductor switch based on the second pulse width modulation signal, characterized in that the driving signal output circuit generates the first switch driving signal and the second switch driving signal at timing at which an off period during which both of the first semiconductor switch and the second semiconductor switch are turned off is set between a period for which the first semiconductor switch is in the on state and a period for which the second semiconductor switch is in the on state.
In the DC-AC conversion apparatus, the driving signal output circuit outputs the first switch driving signal from a point in time of one vertex of the first triangular wave signal to a point in time when the first triangular wave signal immediately after the point in time becomes equal to the feedback signal, and outputs the second switch driving signal from a point in time of the one vertex of the second triangular wave signal to a point in time when the second triangular wave signal immediately after the point in time becomes equal to the feedback signal.
The DC-AC conversion apparatus has a first snubber circuit connected between one end of the primary winding and the second potential point and a second snubber circuit connected between the other end of the primary winding and the second potential point.
The DC-AC conversion apparatus has a first snubber circuit connected between one end of the primary winding and the first potential point and a second snubber circuit connected between the other end of the primary winding and the first potential point.
In the DC-AC conversion apparatus, the first potential point is a contact point to which a power source voltage of the DC power source is applied and the second potential point is a ground.
The invention also provides a parallel running system of DC-AC conversion apparatuses, having a transformer having a primary winding with a center tap and at least one secondary winding, the center tap being connected to a first potential point of a DC power source, a first semiconductor switch connected between one end of the primary winding and a second potential point of the DC power source, a second semiconductor switch connected between the other end of the primary winding and the second potential point, a current detection circuit for detecting a current flowing through a load FL connected to the secondary winding, an oscillation circuit for generating a first triangular wave signal and a second triangular wave signal having a relation in which the first triangular wave signal is reversed when a capacitor for frequency decision and a resistor for frequency decision are connected and also generating a triangular wave signal supplied from the outside and a triangular wave signal having a relation in which the triangular wave signal is reversed when the capacitor for frequency decision and the resistor for frequency decision are not connected, a pulse width modulation circuit for comparing the first triangular wave signal with a feedback signal formed based on a current detected by the current detection circuit and generating a first pulse width modulation signal and comparing the second triangular wave signal with the feedback signal and generating a second pulse width modulation signal, and a driving signal output circuit for alternately outputting a first switch driving signal for turning on the first semiconductor switch based on the first pulse width modulation signal and a second switch driving signal for turning on the second semiconductor switch based on the second pulse width modulation signal, the parallel running system having plural DC-AC conversion apparatuses in which the driving signal output circuit generates the first switch driving signal and the second switch driving signal at timing at which an off period during which both of the first semiconductor switch and the second semiconductor switch are turned off is set between a period for which the first semiconductor switch is in the on state and a period for which the second semiconductor switch is in the on state, characterized in that the capacitor for frequency decision and the resistor for frequency decision are connected to the oscillation circuit in only one of the plural DC-AC conversion apparatuses, and the first DC-AC conversion apparatus having the oscillation circuit to which the capacitor for frequency decision and the resistor for frequency decision are connected supplies only the first triangular wave signal of the first triangular wave signal and the second triangular wave signal generated from the oscillation circuit to the DC-AC conversion apparatuses other than the first DC-AC conversion apparatus, and the plural DC-AC conversion apparatuses perform pulse width modulation control of the same phase synchronously respectively using the first triangular wave signal and the second triangular wave signal.
In the parallel running system of the DC-AC conversion apparatuses, a resistance value of the resistor for frequency decision is set at different values after starting and at the time of starting of the DC-AC conversion apparatus, and the resistance value set at the time of the starting is smaller than the resistance value set after the starting.